The disclosure of Japanese Patent Application No. HEI 11-279402 filed on Sep. 30, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention is directed to emission control apparatuses having three-way catalyst devices for controlling emissions from internal combustion engines.
2. Description of the Related Art
There are known emission control apparatuses having three-way catalyst devices for controlling emissions from internal combustion engines. A three-way catalyst device is capable of simultaneously lessening three components of exhaust gas from an engine, that is, oxides of nitrogen (NOx), hydrocarbons (HC), and carbon monoxide (CO), at high removal rates when the air-fuel ratio of exhaust gas flowing into to the three-way catalyst device (hereinafter, referred to as xe2x80x9cinflow exhaust air-fuel ratioxe2x80x9d) is at a stoichiometric air-fuel ratio. That is, in order to simultaneously lessen the three components, i.e., NOx, HC, and CO, at high removal rates, it is necessary to keep the inflow exhaust air-fuel ratio substantially at the stoichiometric air-fuel ratio.
However, it is very difficult to continuously keep the inflow exhaust air-fuel ratio accurately at the stoichiometric air-fuel ratio. Therefore, in an emission control apparatus for an internal combustion engine disclosed in Japanese Patent Application Laid-Open No. HEI 11-36849, as for example, a three-way catalyst device is provided with an oxygen absorbing/releasing capability of absorbing oxygen when the inflow exhaust air-fuel ratio is on a fuel-lean side of the stoichiometric air-fuel ratio, and of releasing absorbed oxygen when the inflow exhaust air-fuel ratio is on the fuel-rich side of the stoichiometric air-fuel ratio, and the inflow exhaust air-fuel ratio is controlled so that the inflow exhaust air-fuel ratio cyclically changes between the rich and lean sides of the stoichiometric air-fuel ratio and, as a result, the overall inflow exhaust air-fuel ratio becomes substantially equal to the stoichiometric air-fuel ratio. That is, when the inflow exhaust air-fuel ratio is on the rich side of the stoichiometric air-fuel ratio, oxygen absorbed in the three-way catalyst device is released into exhaust gas flowing through the three-way catalyst device, so that the inflow exhaust air-fuel ratio becomes substantially equal to the stoichiometric air-fuel ratio. When the inflow exhaust air-fuel ratio is on the lean side of the stoichiometric air-fuel ratio, oxygen in exhaust gas is absorbed into the three-way catalyst device, so that the inflow exhaust air-fuel ratio becomes substantially equal to the stoichiometric air-fuel ratio.
The three-way catalyst device becomes unable to simultaneously remove the three undesirable components of the exhaust gas when the three-way catalyst device deteriorates, in most cases this means when the oxygen absorbing/releasing capability decreases. Therefore, in the emission control apparatus disclosed in the aforementioned laid-open patent application, deterioration of the three-way catalyst device is evaluated by using outputs from two air-fuel ratio sensors disposed in exhaust passages upstream and downstream of the three-way catalyst device.
That is, the air-fuel ratio sensor disposed upstream of the three-way catalyst device (hereinafter, referred to as xe2x80x9cupstream-side air-fuel ratio sensorxe2x80x9d) outputs an output value corresponding to the air-fuel ratio of exhaust gas that is about to flow into the three-way catalyst device. Since the inflow exhaust air-fuel ratio is controlled so as to cyclically change between the rich and lean sides of the stoichiometric air-fuel ratio as mentioned above, the upstream-side air-fuel ratio sensor alternately outputs relatively high output values corresponding to rich-side air-fuel ratios and relatively low output values corresponding to lean-side air-fuel ratios. On the other hand, the air-fuel ratio sensor disposed downstream of the three-way catalyst device (hereinafter, referred to as xe2x80x9cdownstream-side air-fuel ratio sensorxe2x80x9d) outputs an output value corresponding to the air-fuel ratio of exhaust gas that has passed through the three-way catalyst device. Since the three-way catalyst device has the oxygen absorbing/releasing capability as described above, the three-way catalyst device eliminates relatively small fluctuations, that is, increases and decreases, of the inflow exhaust air-fuel ratio that occur in short cycles. Therefore, the downstream-side air-fuel ratio sensor alternately outputs only relatively high output values corresponding to rich-side air-fuel ratios and relatively low output values corresponding to lean-side air-fuel ratios in fluctuation cycles that are longer than the fluctuation cycles of the output value of the upstream-side air-fuel ratio sensor, as long as the three-way catalyst device operates normally.
However, when the three-way catalyst device has deteriorated, the three-way catalyst device does not eliminate repeated short term increases and decreases in the inflow exhaust air-fuel ratio, so that the output value of the downstream-side air-fuel ratio sensor fluctuates in shorter cycles than when the three-way catalyst device is normal.
Therefore, the emission control apparatus disclosed in the aforementioned laid-open patent application calculates a ratio of the length of an output locus formed by output values of the downstream-side air-fuel ratio sensor (hereinafter, referred to as xe2x80x9cdownstream-side locus lengthxe2x80x9d) to the length of an output locus formed by output values of the upstream-side air-fuel ratio sensor, and determines that the three-way catalyst device has deteriorated when the ratio becomes greater than a predetermined criterion value.
The emission control apparatus disclosed in the aforementioned laid-open patent application is based on an assumption that the downstream-side locus length increases proportionally to the fluctuation of the air-fuel ratio of exhaust gas flowing out of the three-way catalyst device (hereinafter, referred to as xe2x80x9coutflow exhaust air-fuel ratioxe2x80x9d). In reality, however, the downstream-side locus length does not necessarily increase proportionally to the fluctuation of the outflow exhaust air-fuel ratio, due to an output characteristic of the downstream-side air-fuel ratio sensor. More specifically, the amount of change in the output value of the downstream-side air-fuel ratio sensor per unit amount of change in the exhaust air-fuel ratio is relatively small in a range where, for example, the degree of fuel-richness of the exhaust air-fuel ratio is relatively great, and it is relatively great in a range where the degree of fuel-richness of the exhaust air-fuel ratio is relatively small. That is, if the exhaust air-fuel ratio fluctuates within the range where the degree of fuel-richness is relatively great, the change in the downstream-side locus length is relatively small.
In this case, therefore, it is erroneously determined that the three-way catalyst device has not deteriorated, even if the three-way catalyst device has actually deteriorated.
Accordingly, it is an object of the invention to determine deterioration of a three-way catalyst device with good precision.
To achieve the aforementioned and other objects, a catalyst deterioration determining apparatus for an internal combustion engine in accordance with an aspect of the invention includes a three-way catalyst device with an oxygen absorbing/releasing capability that is disposed in an exhaust passage of the engine, and an upstream-side air-fuel ratio sensor disposed in a portion of the exhaust passage located upstream of the three-way catalyst device. The upstream-side air-fuel ratio sensor outputs an output value in accordance with the air-fuel ratio of exhaust gas. Furthermore, a downstream-side air-fuel ratio sensor that outputs an output value in accordance with the air-fuel ratio of exhaust gas is provided in the exhaust passage downstream of the three-way catalyst device. The apparatus calculates a comparison value by comparing the length of an output locus formed by output values of the upstream-side air-fuel ratio sensor and the length of an output locus formed by output values of the downstream-side air-fuel ratio sensor. By comparing the comparison value with a predetermined criterion value, the apparatus determines whether the three-way catalyst device has deteriorated. For the determination, at least one of the downstream-side air-fuel ratio sensor output value and the predetermined criterion value is corrected in accordance with the downstream-side air-fuel ratio sensor output value.
Since an air-fuel ratio varies in its output characteristic in accordance with the output value, the change in the length of the locus formed by output values of the downstream-side air-fuel ratio sensor caused by deterioration of the three-way catalyst device also varies in accordance with the output value of the downstream-side air-fuel ratio sensor. However, since the invention uses a predetermined criterion value or an output value of the downstream-side air-fuel ratio sensor corrected in accordance with the output value of the downstream-side air-fuel ratio sensor, it becomes possible to determine deterioration of the three-way catalyst device with good precision.
In a catalyst deterioration determining method in accordance with another aspect of the invention, a first output value is acquired from an upstream-side air-fuel ratio sensor positioned upstream of a three-way catalyst device disposed in an exhaust gas of an internal combustion engine, the first output value being in accordance with an air-fuel ratio of exhaust gas flowing into the three-way catalyst device. A second output value is acquired from a downstream-side air-fuel ratio sensor positioned downstream of the three-way catalyst device, the second output value being in accordance with the air-fuel ratio of exhaust gas flowing out of the three-way catalyst device. A comparison value is calculated by comparing the length of an output locus formed by the first output value and the length of an output locus formed by the second output value. By comparing the comparison value with a predetermined criterion value, it is determined whether the three-way catalyst device has deteriorated. For the determination, at least one of the second output value of the downstream-side air-fuel ratio sensor and the predetermined criterion value is corrected in accordance with the second output value.